The development of new polymeric materials from renewable resources is gaining considerable attention. Biorenewability is directed toward a sustainable raw material supply where the raw material is renewed from plants or other biological matter, generally through agricultural. Biorenewable polymers are pursued as environmentally friendly replacements for commodity plastics from petrochemical starting materials. The goal is to use low cost readily available starting materials from biorenewable resources such that biorenewable polymers can be competitive with current commercial plastics in the marketplace.
Rather than creating a biorenewable polymer directly from a crop with the limitations imposed by nature with respect to processing and properties, a practical goal is to develop polymers from monomers derived from biorenewable sources that are chemically identical to or a mimic of those derived from petroleum sources. In this manner the market for the biorenewable polymer need not be generated, as materials with the properties to be provided by the new polymers are presently commodities. Additionally, polymerization techniques and processing technologies that are developed along with the monomers for the biorenewable polymers can be designed in light of the methods currently used to produce the commodity polymers.
In general, thermoplastics constitute more than 65% of all global polymer demand and have the possibility to be recycled by melt-processing. Thermoplastic biorenewable polymers are potentially recyclable, which is advantageous for consumer packaging and other high volume needs. A commercially important thermoplastic or its mimic that is prepared by a step-growth process is a particularly practical target for biorenewable monomers.
Step-growth produced polyethylene terephthalate (PET) is the third most common synthetic polymer and accounts for about 20% of world polymer production. This aromatic/aliphatic polyester has very useful thermal properties that are not displayed in an all-aliphatic commodity thermoplastic. PET displays a glass transition temperature (Tg) of 67° C. and a melting temperature (Tm) of 265° C. The key aromatic monomer for preparation of PET, terephthalic acid, is derived from petroleum, and its complementary monomer, ethylene glycol, is derived from petroleum or natural gas.
The design of a sustainable PET mimic requires an aromatic monomer. To this end, an attractive biorenewable source for the aromatic monomer is lignin. Lignin is found in all vascular plants and is the second most abundant naturally-occurring organic polymer, making up approximately 30% of wood. The extraction of lignin from wood is carried out in large scale in the paper pulping industry, and as such, constitutes an attractive source for a PET mimic. In addition to lignin, the bran of rice and maize offers attractive opportunities to harvest potentially useful aromatic/aliphatic monomers. One such monomer, ferulic acid (4-hydroxy-3-methoxycinnamic acid), is found in the cell walls of several plants and is one of the most abundant hydroxycinnamic acids in the plant world. Ferulic acid enhances the rigidity and strength of plants from several families including various grasses such as Graminaceae, vegetable plants from the Solanace family, as well as many flowering plants from the groups of both Monocots as well as Dicots. Seeds of coffee, apple, artichoke, peanut, and orange, as well as both seeds and cell walls of rice, wheat, and oats all contain this phenolic phytochemical. Perhaps the greatest naturally occurring source comes from maize bran, a plant with highly cross-linked cell walls containing esterified ferulic acid, making the structure rather impervious to enzymatic degradation. To this end the preparation of material with comparable thermal properties to polyethylene terephthalate (PET) and polystyrene (PS), two of the most widely used synthetic petroleum derived polymers, based on a functionalized ferulic acid, is a desirable way to achieve the goal of a biorenewable polymer.